1. Field of the Invention
One disclosed aspect of the embodiments relates to an image forming apparatus for bearing toner on a recording material to form an image.
2. Description of the Related Art
As a conventional image forming apparatus, there is a multi-stylus printer that uses a stylus electrode (Japanese Patent Publication No 3-8544). In this multi-stylus printer, an image forming electrode including many stylus electrodes and a cylindrical counter electrode are arranged to face each other with a predetermined gap, and a recording member is conveyed to this gap to contact the image forming electrode. In this state, a voltage corresponding to an image signal is applied to the image forming electrode, and gap discharging is carried out to form a toner image.
The conventional multi-stylus printer using the stylus electrodes in the image forming electrode has a problem, i.e., a stable line width may not be acquired when forming a thin line.
FIG. 12 is a schematic model diagram illustrating a configuration of the conventional image forming apparatus using the stylus electrodes, which includes an image forming electrode 301, a counter electrode 302 bearing toner T, and a recording member 303.
This image forming apparatus uses the gap discharging to bear a toner image on the recording member 303. Specifically, by the gap discharging, charges are supplied from the image forming electrode 301 to a surface opposite a surface of a toner image bearing surface of the recording member 303, and the toner image is retained on the recording member 303 by a coulomb force of the charges. A gap is necessary for generating the gap discharging. A small gap is generated by forming a concave-convex shape on a side of the recording member 303 contacting the image forming electrode 301.
As widely known, a discharging start voltage Vb in a discharging phenomenon in a gap Z can be approximated by the following equation (1) in a gap of 10 μm or more in an atmosphere according to Paschen's law:Vb=312+6.2Z  (1)(Source: p 291 “Electrophotography” by R. M. Shaffert, Kyoritsu Shuppan Co., Ltd.)
In the gap Z, when a potential difference is larger than the discharging start voltage Vb, discharging occurs, and continues until the potential difference is reduced to the discharging start voltage Vb. A surface potential on a non-discharge surface of the recording member 303 after the discharging depends on the gap Z. This means that uneven charging occurs along the concave-convex shape of the recording member 303. In a place where charges are at a lower level in the uneven charging, a holding force of the toner T on the recording member 303 decreases, consequently causing unevenness on the toner image. This unevenness created an unstable line width of the thin line.